Temperature setting on new mod-con boiler/water heater

With help from contributors to this forum, I've had an HTP Phoenix mod-con boiler water heater installed but am now a bit confused about what settings make sense. The unit heats both DHW and a hydronic heating system. I've set the thermostat on the boiler to 130 degrees and the factory setting on the differential is 7 degrees. But I've noticed that the upper tank temperature has dropped as low as 113 degrees and the boiler is still not running at full tilt. The fan speed at that temperature was only 490 RPM while I've seen it run as high as 700 when it's going full bore.

Is it reasonable that there should be a drop of 17 degrees from the setpoint yet the boiler is still not running at top speed to bring the temp back into range? I assume this may have something to do with balancing precise temperature maintenance with energy efficiency. But since 120 degrees is recommended as the minimum temperature for sterilizing dishes, it appears that I may have to either lower the differential or raise the boiler's thermostat to 140-145 to ensure 120 as a minimum. There's a mixing valve on the DHW to prevent scalding but having the higher thermostat setting will compromise my energy efficiency.

Need a little more info. Do you have an indirect WH, or an internal tankless, or what?

The unit I have has an outside reset so it adjusts the desired temp based on that and the return water sensor. I have a tempering valve on my hydronic heating circuit so it never gets the full output of the boiler. So, on mine, it adjusts based on the load, but, if the WH requires heat, it goes to max temp until the demand is over. The WH (an indirect) is on a priority circuit, so the pumps for the radiant won't come on even if the thermostats call for it while it is heating domestic hot water.

The Phoenix is like a boiler with an indirect tank but it's all one unit; the boiler's combustion chamber and heat exchanger are inside the tank with the boiler control unit mounted on the side of the tank (more info here: http://www.htproducts.com/phoenix.html). My radiant heating system pulls water from the tank and uses a heat exchanger to heat the PEX tubing water to about 85 degrees. So far as I know, there's no outside reset such as you have on your system. The radiant is simply controlled with thermostats in each room, turning the flow on or off as needed. The thermostat on the boiler responds to the temperature sensor at the top of the tank and that's the temperature that's displayed on the control panel (although you can also have it tell you the temp at the bottom of the tank as well as fan speed, ionization current and several other readings that I don't fully understand).

I just learned that I have to go out of town tomorrow morning and won't be back until December 10 so I apologize if I can't respond until then.

Not sure which one you have, but you could be losing a fair amount of heat through the heat exchangers, and cooling off the tank. Not sure what the recovery rate is, but it does have a fair amount of capacity. Depending on the length of the pipes to the heat exchangers and the amount of water in them, when the circulator(s) turn on, you'll be dumping some fairly cold water into the tank not counting that being replenished when you are using some at the same time.

Last thing I can think of is to make sure that the water you are pumping out of the tank is coming from the top, and being returned to the cold water input side. You may want a check valve above the point where the domestic water comes off to prevent the possibility of getting some of the cold water from the coils.

The heat exchanger for the radiant flooring is located about eight feet from the unit. All piping is insulated. The house is a large house (7,200 sq. ft.) but only about 60% is heated when we don't have house guests. And the water drawn from the tank for both DHW and radiant is drawn from the top and radiant water returning from the heat exchanger is returned to the bottom of the tank.

Here's some of the specs that might help you to understand the unit's capacities.

Lastly, you said, "You may want a check valve above the point where the domestic water comes off to prevent the possibility of getting some of the cold water from the coils." I'm not sure what this means; can you elaborate?

Let's say the circulation pump is off. that means that there is realtively cold water in the coils. Now you try to get some hot water. Both the cold(er) water from the loop going to the coils and the hot from the tank will both be going to your shower or whatever you turn on. If there is a check valve to force the water to only go one way, then it would have to draw it from the tank rather than being mixed. You must be in a warm place to only need 130K and be heating that much space.

I'll look to see if there appears to be a check valve in the DHW system. As to the capacity of the system, I live in northern California with minimum temps in the high 20's. It remains to be seen whether this new system will work for this house. It was installed this summer and we haven't yet been through a winter. In addition, we have 14 radiant thermostats and keep a number of the rooms at 50 degrees unless we have house guests. We haven't yet had house guests during the cold weather and that'll be the acid test of this system.

I'm still interested in learning if anyone can answer my question about lowering the setpoint of the boiler during the winter when we're on vacation. Anyone?

The lower the set point, the lower the delta T, the less heat can be input into the house. To get a transfer into the zone(s), the water going in has to be hotter than the room, and if too low, it will cool off so very little of the loop is actually hotter than the room, as it would have given off all of the available heat within the first small segment of the loop.

It is a fine line between the length of the loop, the flow rate, and the temperature. Nobody here can tell you exactly, only generally without the exact layout and design (at least from my limited knowledge).